RH2 is an ion exchange resin used to purity water in RO.It can replace Ca2+ in hard water.RH2+Ca2+⟶RCa+2H+Water coming out of ion exchange resin has [H+] = 0.01 M. Thus, hardness of water in ppm Ca2+ ion is

RH2+Ca2+→ RCa+2H+ 1 mol           2 mol ?                  0.01 mol L-1  When H+is 2mol then, Ca2+ is =1mol  When H+is 0.01mol then Ca2+ is =0.012molL−1 ∴Ca2+=0.005M=0.005×40gL−1=0.2gL−11000mL of water has Ca2+=0.2g106mL (parts per million) has Ca2+=0.2×106103=200